Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include portable computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code, system data such as a basic input/output system (BIOS), and other firmware can typically be stored in flash memory devices. Most electronic devices are designed with a single flash memory device.
NAND flash memory devices are becoming popular due to the high memory densities possible at a relatively low cost. The NAND architecture connects 8, 16 or 32 memory cells in series on a single bit line.
NAND flash memory devices can typically be sold to an end user, sold to a memory card manufacturer for combination with a memory controller on memory cards such as MMC cards and SD cards, or combined with a controller by the memory manufacturer to produce a memory card. In any case, the memory device must be tested before the memory device can be sold.
The manufacturing and testing of NAND memory devices is a complicated process. Many tests and functions are needed to make the part reliable. To accomplish this, the memory manufacturers have provided test modes to perform various test functions on the memory device. Some test functions could stress the part beyond its datasheet limits in order to determine if anything will break. These test modes should not be available to the end user of the memory device or the end user of the memory card.
To keep the end user from accessing the test modes, and possibly destroying the part, the manufacturers typically make it difficult to enter these modes. This can be accomplished by using complicated command sequences combined with high voltages on specific pins. This makes it very difficult for end users to enter the test modes since these voltages are not available on the memory cards or end systems. However, this might also make it difficult for some end users to test the parts if they have to generate the high voltages.
If the memory device is sold to an end user, it is desirable to disable any low voltage test mode. If the memory devices are put on cards at the memory manufacturer, it is desirable to have a low voltage test mode available since high voltages are not normally available on the memory card. Also, the testing of the functionality of a NAND flash memory device might be done on a memory card to make the cost more favorable.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved memory device test entry termination.